System for determining and imaging wax deposition and corrosion in pipelines

ABSTRACT

The system for determining and imaging wax deposition and corrosion in pipelines relate to systems for determining wax deposition and corrosion by one or both of two techniques. In both techniques, a source of neutron radiation is directed at the pipeline. In one technique, a neutron detector surrounded by an absorption shield defining a collimation window counts neutrons reflected back to the detector by back diffusion or backscatter radiation. In the other technique, a gamma ray detector measures gamma rays emitted when the emitted neutrons are absorbed in the pipeline. A neutron moderator-reflector is placed around three sides of the pipeline to increase the likelihood of neutron capture. A gamma detector surrounded by a gamma absorption shield defining a collimation window counts neutron capture gamma rays. An energy window can be taken for selection of Fe and H gamma rays for high precision imaging.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/169,691, filed May 31, 2016, which application is acontinuation-in-part of prior U.S. patent application Ser. No.14/717,158, filed May 20, 2015, abandoned, which is a divisional of U.S.patent application Ser. No. 14/497,304, filed Jan. 15, 2014, now U.S.Pat. No. 9,151,722, issued on Oct. 6, 2015.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the determination of the extent ofdeposition of wax and/or corrosion in a pipeline, and particularly to asystem for determining and imaging wax deposition and corrosion in apipeline that uses a neutron source, neutron and/or gamma ray detectors,and backscatter diffusion collimation.

2. Description of the Related Art

Paraffin and asphalt deposition in crude oil transport is a very costlyproblem in oil industry. The accumulation of wax on the inner surface ofpipes reduces flow and may cause a blockage in a pipeline that may stopoil production. Prediction of deposit thickness is very difficult due tothe complex compositions of crude oil. At or below the Wax AppearanceTemperature (WAT), hydrocarbon molecules crystallize and precipitate assolids. Deposition is also a function of system pressure, compositionand pipe inner surface properties. Wax deposition in pipelines can bevery costly for plant operation in the oil industry. New techniques areneeded for allocation and thickness determination of wax deposits. Thetimely removal of wax can produce large savings in the plant'soperational cost.

Thus, a system for determining and imaging wax deposition and corrosionin pipelines solving the aforementioned problems is desired.

SUMMARY OF THE INVENTION

The system for determining and imaging wax deposition and corrosion inpipelines relates to a system for determining wax deposition andcorrosion by one or both of two techniques. In both techniques, a sourceof neutron radiation is directed at the pipeline. In one technique, aneutron detector surrounded by an absorption shield defining acollimation window counts neutrons reflected back to the detector byback diffusion or backscatter radiation. In the other technique, a gammaray detector measures gamma rays emitted when the emitted neutrons areabsorbed in the pipeline. A neutron moderator-reflector is placed aroundthe pipeline to increase the likelihood of neutron capture. These andother features of the present invention will become readily apparentupon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic perspective view of a first embodiment of aneutron detector for system for determining and imaging wax depositionand corrosion in pipelines according to the present invention, showingdetails of the detector's collimator.

FIG. 1B is a diagrammatic perspective view of a second embodiment of aneutron detector for system for determining and imaging wax depositionand corrosion in pipelines according to the present invention, showingdetails of the detector's collimator.

FIG. 1C is a diagrammatic environmental top view of a neutron detectorfor system for determining and imaging wax deposition and corrosion inpipelines according to the present invention, showing positioning of theneutron detector in relation to the pipe to be measured.

FIG. 2 is a schematic diagram of a system for determining and imagingwax deposition and simultaneous corrosion and wax deposit determinationin pipelines according to the present invention, showing the systemconfigured for detecting gamma rays emitted upon neutron capture.

FIG. 3 is a plot showing counts for two minutes of back diffusedneutrons as a function of the thickness of asphalt wax deposited insidea pipeline with a 1 cm wide collimator window along a verticallyoriented neutron detector.

FIG. 4 is a plot showing counts for back diffused neutrons as a functionof window height of the collimator along a vertically oriented neutrondetector.

FIG. 5 is a schematic diagram of an alternative embodiment of a systemfor determining and imaging wax deposition and corrosion in pipelinesaccording to the present invention having a horizontally orientedneutron detector.

FIG. 6 is a plot showing counts for two minutes of paraffin backdiffused neutrons as a function of detector distance d inside the shieldfor the horizontally oriented neutron detector of FIG. 5.

FIG. 7 is a schematic drawing showing the system of FIG. 5 configuredfor vertical scanning of a vertically oriented pipe having increasedthickness of scale (paraffin scale) from top to bottom of the pipe.

FIG. 8 is a plot showing counts of back diffused neutrons from paraffinas a function of detector height and detector distance inside the shieldusing the system of FIG. 7.

FIG. 9 is a plot showing counts of back diffused neutrons frompolyethylene films as a function of the thickness of the polyethylenefilms using different sources of neutron radiation.

FIG. 10 is a plot showing gamma counts for one hour of the 7.63 MeVgamma ray iron double escape peak for 10 cm and 16 cm pipes as afunction of wall thickness measured with the system of FIG. 2.

FIG. 11 is a plot showing simultaneous measurements for 1 hour of thenet gamma counts of the 7.63 MeV iron single escape peak and the 2.23MeV hydrogen peak for a 16 cm diameter pipe with 4 mm wall thickness asa function of paraffin scale thickness using the system of FIG. 2.

FIG. 12 is a schematic drawing of a system for determining and imagingwax deposition and simultaneous corrosion and wax deposit determinationin pipelines according to the present invention, showing the systemconfigured with a gamma collimator for detecting gamma rays emitted uponneutron capture.

FIG. 13 is a plot showing the single escape, double escape and primarygamma ray spectra used, according to the present invention.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system for determining and imaging wax deposition and corrosion inpipelines relate to techniques in which a source of neutron radiation isdirected at the pipeline. In a first technique, a neutron detectorsurrounded by an absorption shield defining a collimation window countsneutrons reflected back to the detector by back diffusion or backscatterradiation. In the other technique, a gamma ray detector measures gammarays emitted when the emitted neutrons are absorbed in the pipeline. Aneutron moderator-reflector like water is placed around three sides,e.g., top, left, and bottom portions of the pipeline to increase thelikelihood of neutron capture. The system will be illustrated in thefollowing examples.

Two types of hydrocarbon scale were investigated, namely, asphalt andparaffin wax, having specific gravities of 2.0 and 0.9, respectively.Actual organic scale is usually a mixture of these materials, and alsocontains small amounts of other molecules (such as molecules containingFe, Ni, Cu, S, Ca, Si, O, etc.), depending on the location of the scalein the plant. In a real inspection, the measurement system needs to becalibrated for the scale type and the specific location. Carbon steelpipes of 16 cm and 10 cm diameters were used for the sample measurementsdescribed herein. These are medium-size pipes commonly used in mostindustrial plants.

The neutron sources used in this work were ²⁴¹Am—Be and ²⁵²Cf ²⁴¹Am—Behas a half-life of 536 yr., and the activity of the material used herewas approximately 1.11×10¹¹ Bq (3Ci), emitting 6.6×10⁶ n/s with atolerance of about ±10%. The bare source gave a neutron dose ofapproximately 6.6×10⁻² mSv/h and a gamma dose of 6.57×10⁻² mSv/h at 1 mdistance, for a total of 0.12 mSv/h. ²⁵²Cf has a 2.64 yr. half-life andemits approximately 5×10⁷ n/s (22 mg or 0.44 GBq). A bare source gives aneutron dose of approximately 0.5 mSv/h and a gamma dose of 0.03 mSv/h,for a total of 0.53 mSv/h. The neutron spectrum of ²⁵²Cf is a fissionspectrum with an average energy of 2.3 MeV, while the ²⁴¹Am—Be has aharder spectrum with an average energy of 3.9 MeV.

The slow neutron detector was a BF₃ gas-filled proportional counter (LNDInc., model 202A, U.S.A). Because the neutron cross section for boron ismuch higher at slow neutron energy, a BF₃ detector exposed to neutronswill respond primarily to slow neutrons. The detector was used with theassociated electronic components of a power supply (type2000 Canberra,Meriden Conn., U.S.A), a preamplifier (type 1406 Canberra, U.S.A), anamplifier (type 2012 Canberra, U.S.A) and an 8192 multi-channel analyzer(PC with special electronic card).

The experimental setup of the neutron back diffusion experiment is shownin FIGS. 1A-1C. As shown in FIGS. 1A and 1B, a cylindrical neutronabsorber or shield 100 is disposed coaxially around a BF₃ neutrondetector 102, and may either have a small, adjustable height collimatorwindow 103 a or an elongate collimator window 103 b extending along thelength of the shield 100. As shown in FIG. 1C, a radiation source 105 isdisposed near the neutron absorber/detector 100, 102 and aimed at thepipeline 115 to be measured. The slit 103 b or window 103 a is aimedlengthwise at a section of the pipe 115 having an organic scale buildup110. Fast neutrons emitted from the source 105 penetrate the iron pipe115 without significant absorption because iron has a small absorptioncross section for fast neutrons. fast neutrons interact elastically andinelastically with hydrogen, carbon atoms and other atoms of the organicscale 110 and are thereby slowed down. Some of the slowed-down neutronswill move backwards and are detected by the BF₃ neutron detector 102,the count rate of which increases with the amount of scale 110. Athicker scale 110 leads to more moderation and backscattering of theslow neutrons.

The BF₃ detector is surrounded with 2.5-cm-thick boron powder thatfunctions as a shield 100 to stop slow neutrons coming directly from thesource 105, or from unwanted neutron interaction with materials otherthan the sample, except for a 1-cm-wide window 103 a facing the pipe110. Different thicknesses of cadmium were also used around the detector102 to stop slow neutrons, and no further reduction in backgroundradiation levels was observed after approximately 1 mm thickness ofcadmium. The background radiation with 5 mm boron reduced three-foldmore than with Cd of approximately the same thickness. Cadmium stopsalmost all neutrons below the cut-off energy of approximately 0.4 eV.Neutrons from the scale with greater energy can penetrate Cd and producecounts in the BF₃ detector, particularly given that the boron crosssection at this energy is not small (approximately 170 b).

The experimental arrangement of the neutron capture gamma ray method isshown in FIG. 2. The same neutron sources (shown in FIG. 2 as 205) wereused together with a high purity germanium detector (HPGe) 118,multichannel analyzer and associated nuclear electronics components. A10-cm-thick layer of paraffin 200 surrounds the source 205 and a10-cm-thick layer of water is place in a vessel such as, for example, Ushaped tube 132 that surrounds the pipe 115, as shown in FIG. 2, tomoderate fast neutrons, increasing the probability of neutron capture atthe pipe position. It should be understood that the 10 cm thicknessdimension of the tube 132 is exemplary only and that other dimensionscommensurate with the dimensional scale of the pipe are contemplated bythe present invention. Similarly, lead blocks 120 are an exemplary tencentimeters thick and are placed between the source 205 and the detector118 to stop gamma rays coming directly from the source 205 and theparaffin moderator 200. Lead blocks 120 are exemplary and it should beunderstood that other gamma shields such as, for example withoutlimitation, tungsten may be used. Lead may also be placed underneath thesetup to stop the capture of gamma rays coming from the concrete floor.Water in concrete contains hydrogen atoms that would emit gamma raysthat interfere with the signal to be measured from the hydrogen atoms ofthe organic scale. The energy of the system can be calibrated usingseveral known sources, such as ¹³⁷Cs (0.662 MeV), ⁶⁰Co (1.332 MeV and1.17 MeV) and neutron capture gamma rays of H (2.223 MeV), Fe (7.632MeV) and C (4.945 MeV).

With respect to neutron back diffusion, plot 300 (shown in FIG. 3)represents the counts for two minutes of back diffused neutrons in thevertically positioned detector 102 surrounded by 2.5 cm boron 100 with a1-cm wide window 103 b along the detector 102 (see FIG. 1B). The averageslope of the curve is 1880 counts/cm for a counting time of two minutes.The median standard deviation is approximately 40. If two counts thatdiffer by 2 standard deviations can be distinguished, then a change inthickness of approximately 0.5 mm can be detected. The sensitivity canbe significantly improved for longer counting times or using a strongersource.

One important aspect of scale inspection is the distribution of thethickness inside the pipe. The measurement illustrated in plot 300 ofFIG. 3 gives the average thickness along the detector. To study thedetection in a smaller region, the detector window facing the pipe wasgradually reduced using the adjustable height window 103 a shown in FIG.1A. Plot 400 of FIG. 4 shows counts from different window heights for anasphalt thickness of approximately 3 cm. Boric acid was also tested asan absorbing agent around the detector. Boric acid (H₃BO₃) is readilyavailable and much less expensive than pure boron. The result in FIG. 4shows that boric acid absorbed fewer neutrons coming directly from thesource 105, compared with the same thickness of pure boron, and gave amuch higher background. Boric acid has a lower atomic concentration ofboron and a lower specific gravity of 1.435 compared with 2.08. However,because of its low cost, it might be considered for applications wherehigh accuracy is not critical.

Another arrangement for the collimation of back diffused neutrons wastested, as shown in FIG. 5. The detector 505 was covered with a 2.5-cmthick layer of boron and laid horizontally, and an opening of 2 cm indiameter was made at the base of the cylindrical absorber 500 fordetector insertion.

The collimation was studied by varying the distance D between theabsorber 500 and the pipe 115 and varying the distance d of the detector505 from the edge of absorber 500. The results, shown in plot 600 ofFIG. 6, demonstrate that collimation improved as both distances wereincreased. Increasing either distance provides a better shield andallows the detector to see a smaller region of pipe and scale. At adistance d of approximately 5 cm (between the detector 505 and the edgeof the absorber 500), the counts are near saturation at all distances Dbetween the shield and pipe. At this value, the detector response to aregion from the pipe 115 is almost equal to the opening window at thebase of the absorber 500. Measurements of paraffin scale of varyingthickness were performed using the experimental setup shown in FIG. 7.The absorber 700, the detector 702 and the source 705 were configuredfor vertical scanning. The thickness of paraffin scale 110 increasedfrom zero at the top of the pipe 115 to almost filling the pipe 115 at aheight of 18 cm from the bottom. Counts at different detector heightsfrom the bottom of the pipe 115 are shown in plot 800 of FIG. 8. Whenthe detector 702 was at the face of the absorber 700 (d=0), nosignificant difference in counts of back diffused neutrons was observedat heights of 12 and 16 cm because the detector 702 saw all scalethicknesses. As the detector height was raised, the scale thicknessdecreased gradually and fewer back diffused neutrons were detected.Between h=16 and h=24 the counts decreased almost linearly. Placing thedetector 702 at distances 5 cm and 10 cm inside the shield 700 yieldedbetter collimation at the expense of total counts. Additionally, as thedistance between the pipe 115 and the detector 702 increased, fewercounts were detected for thicker samples.

The system sensitivity of collimated back diffused neutrons to a changein polyethylene thickness, (in the form of thin sheets) can be obtainedwith reference to plot 900 of FIG. 9. This setup is similar to thatshown in FIG. 5, except that the detector was at the face of the shield(d=0) and the distance between the shield and pipe D was 2 cm Countswere collected for 10 min using both ²⁵²Cf and ²⁴¹Am. The average slopein the middle of the curve is 1600 counts/mm for ten minutes countingtime (2.7 mm⁻¹ s⁻¹), and the standard deviation is approximately 40, sothat a change in thickness of less than one mm in can be detected. Thesensitivity for ²⁵²Cf is 0.54×10⁻⁷ mm⁻¹ per unit neutron strength (2.7mm⁻¹ s⁻¹/5×10⁷ n s⁻¹).

The sensitivity with a ten minute counting time is quite practical forreal inspections. The sensitivity can be improved by using a longcounting time or a stronger source.

The neutron back diffusion method developed herein is rapid andsensitive. As shown in FIG. 3, two minutes of counting was adequate toprovide reliable results. A fraction of a millimeter change in scalethickness can be detected in a very short counting time. The systemcomponents are inexpensive, and the weight of all the components is lessthan 1 kg.

Although the BF₃ detector was quite useful for these measurements,higher efficiency and smaller diameter detectors can be found that mayprovide higher accuracy and efficiency. The pipes used in this work aremedium-sized pipes. Calibration will be needed to enable inspection ofpipes of other sizes.

The boron shield provided better background reduction than the expensiveCd metal. Boric acid is much cheaper than B or Cd, and can be used forneutron absorption if high accuracy is not needed.

Additionally, a detector without an absorber provides both a highersignal and higher background counts. This may be useful for conducting afast survey of scale. A bare detector would respond to other nearbymaterials, such as plastic, wood, moisture, and concrete.

Scale distribution measurements can provide useful information on scaleaccumulation behavior and on the performance of plant components. Thehorizontally positioned detector with collimation from the shield baseprovided more useful information (see FIG. 5 and FIG. 7) than thevertically positioned detector (FIG. 3).). Scale can be measured fromwithin a small portion of the pipe. The reduction in countingsensitivity can always be compensated by extending the counting time.

One important application of neutron capture gamma ray is thesimultaneous measurement of the thicknesses of the iron wall and scale.Wall thickness may reduce with time due to corrosion or erosion.

Plot 1000 of FIG. 10 shows gamma counts for one hour of detection of aniron 7.63 MeV gamma ray double escape peak using an HPGe (high puritygermanium) detector and a ²⁴¹Am—Be source as a function of the wallthickness of the iron pipe for two pipe sizes of 10 and 16 cm, using thesetup shown in FIG. 2. The larger diameter pipe produces higher countsbecause more material is available for interaction. For the 16 cmdiameter pipe, measurements were made when the pipe was empty and whenit was filled with water. The water-filled pipe gave higher countsbecause the water further slows down the neutrons. Counts had an almostlinear relationship with wall thickness. Counts are expected to saturateat much higher iron thicknesses because of the self-absorption of bothincident neutrons and gamma rays emitted from iron. In such industriesas petrochemical or desalination plants, the liquid inside the pipe canbe ordinary water, saline water, or organic liquid. These liquids canhave slight differences in the moderating ratio. Also, in two-phaseflow, the pipe is neither empty nor filled with water. Calibration willbe needed at the actual ratio. If this ratio changes significantly, thenit is better to make measurement when no flow is in the pipe.

Using the same setup, the 7.63 MeV emitted from iron and the 2.23 MeVemitted from the hydrogen in the organic scale were measured at the sametime as a function of asphalt scale thickness. The results are shown inplot 1100 of FIG. 11. Practical applications require the collection of aset of curves similar to those shown in this figure at different pipewall and asphalt thicknesses.

Few studies have been reported in the literature on measurements oforganic scale in pipes or vessels, despite the importance of scaleaccumulation for many industries. A successful online system can savemoney by reducing the frequency of plant shutdowns and the unnecessaryreplacement of components that may still have functional life remaining.

The method described here can work from one side of the object, andtherefore is feasible for scale inspection of large vessels, having verylarge pipes or pipes where only one side can be accessed. The method isalso non-contact, and can work on very hot pipes or tanks.

It should be clarified herein that if the system is to be used foronline inspection, the flow of organic fluid needs to be stopped so thatsignals from the fluid do not interfere with signals from the scale. Themain advantage of neutron capture gamma-ray over neutron back diffusionis that it provides simultaneous information on both the scale thicknessand the corrosion or thickness of the pipe wall (FIG. 10 & FIG. 11).Portable HPGe detectors and multichannel analyzers are commerciallyavailable for field work, but this equipment is expensive and the setupis more complicated compared to the setup for neutron back diffusion.

Paraffin was used as the neutron moderator around the source in theneutron capture method. While polyethylene, for example, has a highermoderating ratio than paraffin and a higher melting point and is morepractical for field work, it is much more expensive. This considerationalso applies to the choice of water as a neutron moderator and reflectoraround the pipe. The moderator thicknesses were selected based on anoptimization of the geometrical setup, although there might be some roomfor further improvements in geometry.

Many radioactive neutron sources can be used. The two sources studiedhere can achieve the goals of this work. The ²⁵²Cf source has arelatively short half-life of 2.64 y, so that frequent correction orcalibration will be needed. This source also gives less radiation doseper unit strength than ²⁴¹Am—Be. Higher activity sources can be used formore accurate or faster measurements. Neutron sources with a much higheractivity are used for field applications, such as oil well logging. Inindustrial gamma radiography, sources of approximately 100 Ci ¹⁹²Ir areused, and they are bare during imaging.

Such a source gives a dose of 420 mSv/h at 1 m, much higher than thedose given by the neutron sources used here of less than 1 mSv/h. Ashield with source remote control can also significantly reduce the dosein all field applications.

In a further embodiment, as shown in FIG. 12, a gamma collimator 1202 isdisposed in front of the detector 118, the collimated gamma rays 1204impinging the detector 118. In this case the detector 118 will detectradiation coming from only a small collimation region 1206 of the pipe115. Moreover, the whole system can be put into rotation to rotatearound a circumference of pipe 115 (via a rotating structure, or if thedetector is handheld via a handheld rotational sweep) such that only asmall region of pipe 115 and scale can be inspected at a time. Imagingand profile of the corrosion and scale can be made. For this purpose, asshown in plot 1300 of FIG. 13, energy windows can be taken across the7.63 MeV Fe primary, 7.119 MeV Fe single escape and 6.608 MeV Fe doubleescape. Each window can be taken to construct an image of the iron wallpipe. The three images can be taken in coincidence to produce a highprecision single image. Similarly plot 1300 illustrates that images canbe taken for the scale by taking windows across the 2.22 MeV H primary,1.709 MeV H single escape and 1.198 MeV H double escape to constructprofile and images of scale. Accordingly two separate images ofcorrosion in pipe wall and scale accumulation can be obtained.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims.

1. A system for determination of wax deposition and corrosion in apipeline, comprising: a neutron radiation source for emitting neutronstowards a pipeline; a slow neutron detector; an absorption shield havinga high slow neutron absorption cross section surrounding the slowneutron detector, the absorption shield defining a collimation windowfor collimating neutrons diffusing back to the detector from thepipeline; and means for counting neutrons diffusing back to the slowneutron detector, the amount of wax deposition and corrosion in thepipeline being positively correlated with the count of neutrons asprovided by the counting means.
 2. The system for determination of waxdeposition and corrosion in a pipeline according to claim 1, furthercomprising means for calibrating the system before the system is used todetermine wax deposition and corrosion in the pipeline.
 3. The systemfor determination of wax deposition and corrosion in a pipelineaccording to claim 1, wherein composition of the absorption shieldsurrounding the slow neutron detector is selected from the groupconsisting of a thick layer of boron powder, cadmium, and boric acid. 4.The system for determination of wax deposition and corrosion in apipeline according to claim 1, wherein the absorption shield surroundingthe slow neutron detector comprises a 2.5 cm thick layer of boronpowder.
 5. The system for determination of wax deposition and corrosionin a pipeline according to claim 1, wherein the counting means providesa proportional count of the neutrons detected at the slow neutrondetector.
 6. The system for determination of wax deposition andcorrosion in a pipeline according to claim 1, wherein the collimationwindow aimed at the pipeline is 1 cm wide.
 7. The system fordetermination of wax deposition and corrosion in a pipeline according toclaim 1, wherein the collimation window is adjustable in height.
 8. Thesystem for determination of wax deposition and corrosion in a pipelineaccording to claim 1, wherein said detector is vertically oriented. 9.The system for determination of wax deposition and corrosion in apipeline according to claim 1, wherein the neutron radiation source isselected from the group consisting of 241Am—Be and 252Cf.
 10. The systemfor determination of wax deposition and corrosion in a pipelineaccording to claim 1, wherein said detector is oriented horizontally.11. The system for determination of wax deposition and corrosion in apipeline according to claim 10, wherein said absorption shield iscylindrical, collimation being adjustable by adjusting distance betweensaid detector and the pipeline and by retracting said detector aselectable distance within said cylindrical absorption shield. 12-25.(canceled)